svn commit: r248196 - head/sys/nfs

[Andre Oppermann]

On 12.03.2013 13:19, Gleb Smirnoff wrote:
> Author: glebius
> Date: Tue Mar 12 12:19:23 2013
> New Revision: 248196
> URL: http://svnweb.freebsd.org/changeset/base/248196
>
> Log:
>    Use m_get2() to get mbuf of appropriate size.

The problem with m_get2() is that it will attempt to use jumbo mbufs
larger than PAGE_SIZE if the request size is sufficiently large.

In light of the recent issues with jumbo > PAGE_SIZE allocations on
loaded systems I don't think it is appropriate to extend this practice
further.

Anything that needs more than PAGE_SIZE (where PAGE_SIZE == 4K) mbuf
space should allocate an mbuf chain with m_getm2().  That is what we
have mbuf chains for.

Sufficient availability of mbufs > PAGE_SIZE cannot be guaranteed after
some uptime and/or a loaded system due to memory fragmentation.  Allocation
can fail non-deterministically for mbufs > PAGE_SIZE long before smaller
mbufs become unavailable due to overall (kernel) memory exhaustion.

Mbufs > PAGE_SIZE are not only contiguous in kernel address space but
also in real memory address space.  They are intended jumbo frames on
poor NIC's without scatter-capable DMA engines.

When you put the m_get2() function proposal for review and comments I
already highlighted these issues and put forward serious concerns about
this.

Please change m_get2() to limit itself to mbuf allocations <= PAGE_SIZE.

-- 
Andre

Please note that I applaud your general clean up of mbuf related allocation
functions and users thereof. :-)